An alkali fusion process has been applied for a long time to the production of aromatic hydroxy compounds from alkali salts of aromatic sulfonic acids by reacting the Starting material with an alkali metal hydroxide. This process, however, has a disadvantage in that agitation and mixing of the reaction materials become difficult as the reaction proceeds, as well as a foaming problem, because the reaction mixture changes its phase from liquid to powder via a slurry phase and a high viscosity phase with the progress of the reaction.
As countermeasures to overcome such problems, several processes have been proposed, for example, (1) a process in which a high power agitation apparatus or a mixing apparatus of a multiblade type is used (JP-B 33-9970), (2) a process in which an alkali material is added to the reaction mixture with the aim of keeping the reaction in powder phase throughout the entire reaction period (JP-A 51-52142), (3) a process in which an alkali material is used in an excess amount to fluidize the reaction mixture and (4) a process in which the reaction is effected under a pressured condition in the presence of water. (The terms "JP-B" and "JP-A" as used herein mean an "examined Japanese patent publication" and an "unexamined published Japanese patent application", respectively) These prior art processes, however, still have disadvantages. For instance, the above processes 1 and 4 require special equipments which entail expensive facility costs, the process 2 requires complex handling because not only it is difficult to keep the reaction mixture in powder form but also it is necessary to modify the agitation system and the process 3 cannot prevent foaming even by the use of an excess amount of the alkali-material in addition to a necessity to use a large quantity of acid to neutralize the alkali material remained in a large quantity after the reaction and Such a neutralization step generates a large volume of waste water which causes a problem of requiring additional treatment cost.
In consequence, for the purpose of improving fluidity of a reaction mixture in the alkali fusion reaction, further modified processes have been proposed in which reaction of an alkali salt of an aromatic sulfonic acid with an alkali metal hydroxide is carried out by uniformly dispersing in an inert solvent as a reaction medium. For example, utilization of kerosine or paraffinic hydrocarbons as the reaction medium has been disclosed in U.S. Pat. No. 2,111,973 and British Patent 181,673, as well as by Yura and Matsui in Journal of Industrial Chemistry (Kogyo Kagaku Zasshi in Japanese, vol. 47, p. 817). As another example, the use of aliphatic, alicyclic hydrocarbons, aromatic hydrocarbons or aromatic ethers as the reaction medium has been disclosed in JP-B 2-35732 (also in U.S. Pat. No. 4,633,024 and European Patent 92,772 as declaration of priority with regard to the Japanese patent; the U.S. Patent discloses only a limited examples of aromatic hydrocarbons).
Even in the case of these prior art processes in which inert solvents are used as the reaction medium, an alkali salt of aromatic sulfonic acid and an alkali metal hydroxide to be used as the starting materials cannot be dispersed to a satisfactory level in some of these reaction media, or do not disperse at all in some cases, thus resulting in the separation of these starting materials and the reaction medium from one another and subsequently in the adhesion of the separated compounds to the wall and impeller blades in a reactor vessel. For example, even when the inert solvents disclosed in the aforementioned U.S. Pat. No. 2,111,973 and JP-B 2-35732 (also in U.S. Pat. No. 4,633,024 and European Patent 92,772 as declaration of priority with regard to the Japanese patent) are used, high yield production of an aromatic hydroxy compound cannot be attained because of insufficient reaction due to phase separation of the reaction mixture. In addition to these problems, because of markedly different temperature levels between the reactor wall and the contents in the reactor owing to insufficient dispersion, it is difficult to control the reaction temperature within an optimum range and the unstable temperature control results in tar formation. Also, because of these problems, a reaction mixture cannot be discharged smoothly from a reactor vessel and therefore a continuous reaction system cannot be accomplished.
For the purpose of overcoming these problems involved in the prior art, the inventors of the present invention have conducted intensive studies on the reaction of an aromatic sulfonic acid alkali metal salt and/or an aromatic sulfonic acid with an alkali metal hydroxide in various reaction media and found that interfacial tension between a reaction medium and a melted product of alkali metal hydroxide was taking part in the phenomena which reveals the dispersion of the reaction mixture or non-dispersion of the reaction mixture being adhered to the wall of the reactor vessel. Since then, the present inventors have examined various techniques to reduce such an interfacial tension and found that the alkali fusion reaction can be effected in completely uniform dispersion phase by the use of a small amount of a surface active agent even in the case of the use of a reaction medium which usually causes adhesion of reaction materials to the wall and impeller blades in a reactor and subsequently impedes operations of the reaction. The present invention has been accomplished as a result of these efforts. An alkali fusion reaction is carried out at a high temperature under strong alkaline conditions. Astonishingly, the present inventors have found that even in such a hard reaction conditions a wide variety of surface active agents is effective in the process of this invention.